Method for compensating shrinkage of plastic webs

ABSTRACT

When producing plastic cards, a plurality of layers is laminated under application of heat and pressure resulting in unpredictable shrinking. In order to compensate shrinking, the laminated web is elongated to adapt to its nominal alignment size. Elongation is produced by material displacement along grid bars separating to-be-fabricated cards later punched off the web leaving a waste grid.

BACKGROUND OF THE INVENTION

The present invention relates to a method for processing plasticmaterial webs, the webs being provided with card-shaped images separatedby a blank grid. Upon completion of the processing, the individualimages will be punched from the web thereby producing credit cards,telephone cards, smart cards, and the like. The remaining blank grid isdiscarded.

Processing of such webs must meet extreme accuracy requirements. Forexample, when milling a recess into a card body for accommodation of achip, the site of the recess must not deviate from predeterminedcoordinates relative to the card edges.

Frequently, such webs or cards are produced as multi layer laminates. Ina laminating installation, the layers are hot pressed and then cooleddown. A web so produced undergoes a shrinking process, that is, thedimensions of the web in longitudinal and transverse directionsdecrease. Even if the shrinking of, say, a few tenths of a millimeteramounts to only tenths of a percent when related to the standard cardlength of 86 millimeter, it nevertheless detrimentally affects theoperations in following processing stations. Misalignments and/ordistortions of the printed image on the cards may result from shrinking.

In a known plastic card fabrication process, positioning holes areprovided in the blank grid for cooperation with positioning pins locatedat the various processing stations so as to present the to-be-fabricatedcards in proper alignment. While the distance reduction due to shrinkingmay be insufficient to bring immediately succeeding holes off thecapturing range of the pins, this may not hold true if the pins areallocated to each second hole or have even greater spacing.

The shrinking problem is aggravated additionally because it is not afixed parameter but may vary depending upon the used material and mayeven vary when a fresh batch of one layer is processed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a card manufacturingprocess wherein the detrimental effects of web shrinking are overcome.In accordance with the present invention, the web which has beensubjected to shrinking is stretched in a unique manner to compensate theshrinking. It is not the entire web which is elongated but onlytransverse blank grid bars are subjected to cold deformation in thatindentations are produced therein thereby dislodging material adjacentthose indentations such that the web is elongated to adapt to itsnominal alignment size. The amount of material displacement and thus theamount of elongation may advantageously be controlled by the penetrationdepth of pins pressed into the transverse grid bars. As the amount ofshrinking is hardly predictable, preferably the actual spacing ofsuccessive holes (or other marks, e.g. printed cross-hairs) is measuredand compared with the nominal spacing so as to form a servo loop(feedback control) with drive means determining the penetration depth.Optionally, the pins may produce a cold deformation or they may beheated, depending on the particular material. Indentation may beprovided on only one face of the web or on both faces, and optionally,the indentations may form through holes. Depending upon the needs, eachtransverse grid bar or each second, third, . . . , one may be providedwith the indentations of the invention.

German published patent application DE 36 12 518 A1 discloses a processwherein a plastic body is produced e.g. by means of injection molding.The body so formed is subjected to a cold deformation so that it may fiton some other member. The body is then heated and tends to assume itsoriginally molded shape. For example, a bottle cap is produced, thenwidened so that it fits over a bottle mouth, and heated to shrinkthereon. It will be appreciated that the problem underlying the teachingof '518 as well as the steps taught to solve it are quite different fromthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view, partly broken away, of a plastic materialtape after being stretched in accordance with the teaching of theinvention.

FIG. 2 is a schematic plan view of a portion of a stretched plastic web.

FIG. 3 is a similar plan view of a stretched sheet.

DESCRIPTION OF THE SPECIFIC EMBODIMENT

Embodiments of the invention are illustrated in the attached drawingsand will now be explained with reference thereto.

FIG. 1 shows a portion of a plastic material tape or web 10 placed on aplane support 12. Two to-be-fabricated cards 14 and 16 are indicated bytheir contours although they will be punched from the web at a laterstage. Between adjacent cards there is a blank grid comprisingtransverse bars 18 and longitudinal bars 19. Indentations 22 separatedby spacers 24 have been pressed into transverse bar 18 by means of atool 20 which is reciprocable orthogonally to the plane of support 12 asindicated by the arrow. The tool 20 comprises a plurality oftransversely spaced apart indentation tips, usually being tapered in thedownward direction. Thus, a plurality of spaced indentations 22 areprovided between the "cards" 14, 16. Preferably, indentations 22 andspacers 24 are provided along rows and/or columns. It is preferred thatthe indentations taper in penetration direction as may be seen at thesectioned indentation. Thus, the penetration depth determines the amountby which the portions of the web containing the to-be-fabricated cardsare spread apart. The illustrated tool shape and resulting shape ofindentations is only one of a greta variety of possible shapes. Forexample, the indentations could be cone shaped or frustoconical.

In another embodiment, not illustrated, indentations may be producedfrom both faces of the web, and the indentations of one and the same rowmay alternatingly be produced on the front face and the back face.

Formation of the indentations 22 physically displaces material in alongitudinal direction so that the web as a whole is elongated. It ispreferred to use cold deformation but, optionally, tool 20 could beheated. When cold deformation is applied, care must be taken not toexceed the breaking load which for particular materials, such aspolyvinylchloride (PVC), is quite close to the deformation load.Therefore, the individual indentations are separated by spacers 24. Inother materials, these indentations comprise a continuous grooveextending along most or the entire length of the blank grid bar 18.

FIG. 2 illustrates a somewhat longer portion of web 10. A plurality ofto-be-fabricated cards 15 are arranged in successive pairs. The web 10is provided with positioning holes 26. In a preferred embodiment, theactual distance separating adjacent holes 26 is measured and comparedwith a nominal distance, and the penetration depth of tool 20 iscontrolled to minimize the difference between actual and nominaldistances.

The invention is not limited to elongation of tape-like webs. Asillustrated in FIG. 3, sheets 27 having rows and columns ofto-be-fabricated cards 17, may be elongated in at least one direction,as indicated by the indentations 22. In a particular fabricationprocess, such sheets are cut to form strips similar to the web shown inFIG. 2, the strips are chained to form such a web, and positioning holesare punches through. The processing will then proceed as set forthabove.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

What is claimed is:
 1. A method for processing a plastic material webprovided with card-shaped images which are separated by a blank grid,said material having undergone a shrinking process to produce a shrunkmaterial, comprising elongating said shrunk material by producingindentations in said blank grid whereby material thereof is displaced ina direction of elongation and said card-shaped images to compensate forsaid shrinking process.
 2. The method of claim 1 wherein a predeterminednumber of adjacent indentations is produced along a grid bar.
 3. Themethod of claim 1 wherein said indentations are produced by cold plasticdeformation.
 4. The method of claim 3 wherein separate indentations areproduced along a grid bar.
 5. The method of claim 1 wherein indentationtips are pressed into said blank grid.
 6. The method of claim 5 whereinan amount of elongation is determined by a tip penetration depth.
 7. Themethod of claim 1 wherein said web is tape-shaped.
 8. The method ofclaim 7 wherein said web is provided with nominally equidistant indexmarks and wherein an amount of elongation is automatically controlled inresponse to an actually measured index mark spacing.
 9. The method ofclaim 8 wherein said index marks are holes punched through said web.